Chemical vapor deposition (CVD) is a chemical process that is used to deposit solid materials on substrates, and is commonly employed in the manufacture of semiconductor devices. In chemical vapor deposition processes, a substrate is exposed to one or more reagent gases, which react, decompose, or both react and decompose in a manner that results in the deposition of a solid material on the surface of the substrate.
One particular type of CVD process is referred to in the art as vapor phase epitaxy (VPE). In VPE processes, a substrate is exposed to one or more reagent vapors in a reaction chamber, which react, decompose, or both react and decompose in a manner that results in the epitaxial deposition of a solid material on the surface of the substrate. VPE processes are often used to deposit III-V semiconductor materials. When one of the reagent vapors in a VPE process comprises a halide vapor, the process may be referred to as a halide vapor phase epitaxy (HVPE) process.
It is known in the art to form III-nitride semiconductor materials, such as gallium nitride (GaN), using VPE processes in which metallorganic (MO) precursor materials are decomposed within a reaction chamber to form the III-nitride semiconductor material. Such processes are often referred to as metallorganic vapor phase epitaxy (MOVPE) processes, and may also be characterized as metallorganic chemical vapor deposition (MOCVD) processes. Such MOVPE processes are commonly performed utilizing several sequential pre-deposition processes prior to the deposition of the desired bulk III-nitride semiconductor material. These sequential pre-deposition processes may include a high temperature hydrogen bake of the growth substrate (e.g., a sapphire substrate), nitridation of the growth substrate, formation of a nucleation template layer of a III-nitride material at relatively low temperatures on the growth substrate, annealing of the nucleation template layer at relatively higher temperatures, coalescence of the nucleation template layer, and finally growth of the bulk III-nitride material layer on the nucleation template layer.
HVPE processes are also used to form III-nitride semiconductor materials such as gallium nitride (GaN). In such processes, epitaxial growth of GaN on a substrate may result from a vapor phase reaction between gallium chloride (GaCl) and ammonia (NH3) that is carried out within a reaction chamber at elevated temperatures between about 500° C. and about 1,000° C. The NH3 may be supplied from a standard source of NH3 gas. In some methods, the GaCl vapor is provided by passing hydrogen chloride (HCl) gas (which may be supplied from a standard source of HCl gas) over heated liquid gallium (Ga) to form GaCl in situ within the reaction chamber. The liquid gallium may be heated to a temperature of between about 750° C. and about 850° C. The GaCl and the NH3 may be directed to (e.g., over) a surface of a heated substrate, such as a wafer of semiconductor material. U.S. Pat. No. 6,179,913, which issued Jan. 30, 2001 to Solomon et al., discloses a gas injection system for use in such systems and methods.
HVPE processes are currently widely used to grow relatively thick GaN layers on sapphire, mainly due to the relatively fast growth rates that can be attained through HVPE processes, which growth rates range from tens to hundreds of microns per hour. The growth of thick GaN layers using HVPE, however, usually requires GaN template layers grown by metal-organic chemical vapor deposition (MOCVD). Without such GaN template layers, GaN layers grown directly on sapphire usually crack when thickness exceeds certain values.